In this work, we developed a method, which is used for approximation of basic magnetic field B0 inside a specific
spherical volume. This method uses Legendre multinominals expansion of magnetic flux density field measured at discrete points on
spherical volume. Using optimization technique of Least square method we modeled map of magnetic flux density on the desired
surface and compared it to measured values of magnetic flux density. The coefficients of spherical function expansion obtained from
approximation process can be used for determination of shimming currents which feed correction coils of tomograph. The results of
approximated maps are described and compared to measured maps in the experimental part of this article.